The answer

Earth's air is about 20% oxygen. It has never had much more than this, as far as we can tell. It certainly has never been 50%. Keep in mind that pure oxygen is a very dangerous and reactive gas, and is like a poison in the way that it is a very strong oxidizer--too much of a good thing. Materials can just burst into flames in pure oxygen. So 20% is a nice amount that "life" can handle safely. In the very early history of the Earth, even before there was life, the atmosphere was Helium and Hydrogen, then a couple billion years later it was mostly Carbon Dioxide and other things. Finally, when plants appeared a few billion years ago, oxygen began forming in the atmosphere. You can learn more at the Wikipedia.

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